4,873 research outputs found
Oscillating Superfluidity of Bosons in Optical Lattices
We follow up on a recent suggestion by C. Orzel et. al., Science, 291, 2386
(2001), whereby bosons in an optical lattice would be subjected to a sudden
parameter change from the Mott to the superfluid phase. We analyze the Bose
Hubbard model with a modified coherent states path integral which can escribe -
both - phases. The saddle point theory yields collective oscillations of the
uniform superfluid order parameter. These would be seen in time resolved
interference patterns made by the released gas. We calculate the collective
oscillation's damping rate by phason pair emission. In two dimensions the
overdamped region largely overlaps with the quantum critical region.
Measurements of critical dynamics on the Mott side are proposed.Comment: 4 pages 1 eps figures; Final version as appears in PRL. Added
discussion on spontaneous generation of vortice
Optimal T of cuprates: role of screening and reservoir layers
We explore the role of charge reservoir layers (CRLs) on the superconducting
transition temperature of cuprate superconductors. Specifically, we study the
effect of CRLs with efficient short distance dielectric screening coupled
capacitively to copper oxide metallic layers. We argue that dielectric
screening at short distances and at frequencies of the order of the
superconducting gap, but small compared to the Fermi energy can significantly
enhance T, the transition temperature of an unconventional superconductor.
We discuss the relevance of our qualitative arguments to a broader class of
unconventional superconductors.Comment: 8 Pages, 4 figure
Approximation and geometric modeling with simplex B-splines associated with irregular triangles
Bivariate quadratic simplical B-splines defined by their corresponding set of knots derived from a (suboptimal) constrained Delaunay triangulation of the domain are employed to obtain a C1-smooth surface. The generation of triangle vertices is adjusted to the areal distribution of the data in the domain. We emphasize here that the vertices of the triangles initially define the knots of the B-splines and do generally not coincide with the abscissae of the data. Thus, this approach is well suited to process scattered data.\ud
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With each vertex of a given triangle we associate two additional points which give rise to six configurations of five knots defining six linearly independent bivariate quadratic B-splines supported on the convex hull of the corresponding five knots.\ud
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If we consider the vertices of the triangulation as threefold knots, the bivariate quadratic B-splines turn into the well known bivariate quadratic Bernstein-BĂ©zier-form polynomials on triangles. Thus we might be led to think of B-splines as of smoothed versions of Bernstein-BĂ©zier polynomials with respect to the entire domain. From the degenerate Bernstein-BĂ©zier situation we deduce rules how to locate the additional points associated with each vertex to establish knot configurations that allow the modeling of discontinuities of the function itself or any of its directional derivatives. We find that four collinear knots out of the set of five defining an individual quadratic B-spline generate a discontinuity in the surface along the line they constitute, and that analogously three collinear knots generate a discontinuity in a first derivative.\ud
Finally, the coefficients of the linear combinations of normalized simplicial B-splines are visualized as geometric control points satisfying the convex hull property.\ud
Thus, bivariate quadratic B-splines associated with irregular triangles provide a great flexibility to approximate and model fast changing or even functions with any given discontinuities from scattered data.\ud
An example for least squares approximation with simplex splines is presented
Spin 3/2 dimer model
We present a parent Hamiltonian for weakly dimerized valence bond solid
states for arbitrary half-integral S. While the model reduces for S=1/2 to the
Majumdar-Ghosh Hamiltonian we discuss this model and its properties for S=3/2.
Its degenerate ground state is the most popular toy model state for discussing
dimerization in spin 3/2 chains. In particular, it describes the impurity
induced dimer phase in Cr8Ni as proposed recently. We point out that the
explicit construction of the Hamiltonian and its main features apply to
arbitrary half-integral spin S.Comment: 5+ pages, 6 figures; to appear in Europhysics Letter
Effect of anisotropy on the field induced quantum critical properties of the three dimensional s=1/2 Heisenberg model
The field induced quantum critical properties of the three dimensional
spin-1/2 anisotropic antiferromagnetic Heisenberg model has been studied. We
have investigated the quantum phase transition between the spiral order and
field induced ferromagnetic order by means of Bose-Einstein condensation of
magnons in terms of a bosonic representation. The effect of in-plane anisotropy
on the critical properties has been studied via the bosonic model by Green's
function approach. We have found an analytic expression for the gap exponent in
addition to numerical results for the critical magnetic field in terms of
anisotropy parameter. The in-plane anisotropy breaks the U(1) symmetry
explicitly which changes the universal behavior by a drastic change on the gap
exponent. Moreover, the critical magnetic field depends strongly on the
in-plane anisotropies. The divergence of the transverse structure factor at the
antiferromagnetic wave vector confirms the onset of the magnetic order which
scales with the negative value of gap exponent as the magnetic field approaches
the critical one. The transverse staggered magnetization as an order parameter
vanishes with exponent when the magnetic field reaches its critical
value in low field region.Comment: 9 pages and 2 figure
Control of gradient-driven instabilities using shear Alfv\'en beat waves
A new technique for manipulation and control of gradient-driven instabilities
through nonlinear interaction with Alfv\'en waves in a laboratory plasma is
presented. A narrow field-aligned density depletion is created in the Large
Plasma Device (LAPD), resulting in coherent unstable fluctuations on the
periphery of the depletion. Two independent kinetic Alfv\'en waves are launched
along the depletion at separate frequencies, creating a nonlinear beat-wave
response at or near the frequency of the original instability. When the
beat-wave has sufficient amplitude, the original unstable mode is suppressed,
leaving only the beat-wave response at a different frequency, generally at
lower amplitude.Comment: Submitted for Publication in Physical Review Letters. Revision 2
reflects changes suggested by referees for PRL submission. One figure
removed, several major changes to another figure, and a number of major and
minor changes to the tex
Floquet Spectrum and Transport Through an Irradiated Graphene Ribbon
Graphene subject to a spatially uniform, circularly-polarized electric field
supports a Floquet spectrum with properties akin to those of a topological
insulator, including non-vanishing Chern numbers associated with bulk bands and
current-carrying edge states. Transport properties of this system however are
complicated by the non-equilibrium occupations of the Floquet states. We
address this by considering transport in a two-terminal ribbon geometry for
which the leads have well-defined chemical potentials, with an irradiated
central scattering region. We demonstrate the presence of edge states, which
for infinite mass boundary conditions may be associated with only one of the
two valleys. At low frequencies, the bulk DC conductivity near zero energy is
shown to be dominated by a series of states with very narrow anticrossings,
leading to super-diffusive behavior. For very long ribbons, a ballistic regime
emerges in which edge state transport dominates.Comment: 4.2 pages, 3 figure
Instability of charge ordered states in doped antiferromagnets
We analyze the induced interactions between localized holes in weakly-doped
Heisenberg antiferromagnets due to the modification of the quantum zero point
spin wave energy; i.e. the analogue of the Casimir effect. We show that this
interaction is uniformly attractive and falls off as r^{-2 d+1} in d
dimensions. For ``stripes'', i.e parallel (d-1)-dimensional hypersurfaces of
localized holes, the interaction energy per unit hyperarea is attractive and
falls, generically, like r^{-d}. We argue that, in the absence of a long-range
Coulomb repulsion between holes, this interaction leads to an instability of
any charge-ordered state in the dilute doping limit.Comment: Revtex, 5 pages two-column format, 3 ps figures (epsf). Two
references added and some textual change
Low-Temperature Properties of Two-Dimensional Ideal Ferromagnets
The manifestation of the spin-wave interaction in the low-temperature series
of the partition function has been investigated extensively over more than
seven decades in the case of the three-dimensional ferromagnet. Surprisingly,
the same problem regarding ferromagnets in two spatial dimensions, to the best
of our knowledge, has never been addressed in a systematic way so far. In the
present paper the low-temperature properties of two-dimensional ideal
ferromagnets are analyzed within the model-independent method of effective
Lagrangians. The low-temperature expansion of the partition function is
evaluated up to two-loop order and the general structure of this series is
discussed, including the effect of a weak external magnetic field. Our results
apply to two-dimensional ideal ferromagnets which exhibit a spontaneously
broken spin rotation symmetry O(3) O(2) and are defined on a square,
honeycomb, triangular or Kagom\'e lattice. Remarkably, the spin-wave
interaction only sets in at three-loop order. In particular, there is no
interaction term of order in the low-temperature series for the free
energy density. This is the analog of the statement that, in the case of
three-dimensional ferromagnets, there is no interaction term of order in
the free energy density. We also provide a careful discussion of the
implications of the Mermin-Wagner theorem in the present context and thereby
put our low-temperature expansions on safe grounds.Comment: 24 pages, 3 figure
Widths of Isobaric Analog Resonances: a microscopic approach
A self-consistent particle-phonon coupling model is used to investigate the
properties of the isobaric analog resonance in Bi. It is shown that
quantitative agreement with experimental data for the energy and the width can
be obtained if the effects of isospin-breaking nuclear forces are included, in
addition to the Coulomb force effects. A connection between microscopic model
predictions and doorway state approaches which make use of the isovector
monopole resonance, is established via a phenomenological ansatz for the
optical potential.Comment: 18 pages, 1 figure. To appear on Phys. Rev. C (tentatively scheduled
for June 1998
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